Nidah 56b - Kusi'im and their beliefs

The Mishna says that Kusim are believed to say that there is no miscarried fetus buried in a particular location. In general we assume that they are believed on d'oryasa things but not d'rabonon. Areas of halacha that they keep, Rav Shimon Ben Gamliel in Gittin 10a says that they are more strict than Jews in the way they keep it. However, they generally don't accept Rabbinic law (with the exception of cases where the rabbonon used a pasuk as an esamachta for their institution, and the kusim believed it to be the simple understanding of the pasuk as the gemara says 57a by tziyun kevaros), and they also reject the interpretation of chazal that remove the pesukim from their simple understanding. This is why the gemara assumes that they wouldn't darshen the pasuk of לפני עור לא תתן מכשול to include causing someone to do an aveira. They take the pasuk literally to refer to placing a stumbling block in front of a blind man as rashi writes in chulin (it is unclear whether they would include giving bad practical advice in this pasuk). Therefore, the Kusi is only believed that a place is tahor if he is a kohein standing on that place, otherwise it would just be an issue of causing someone else to stumble which they aren't sensitive to.

The main issue with Kusim is that they don't trust the Rabbonon in their assumptions and drashos. Tosafos asks why are kusim believed about their kesamim, since the tu'mah of a kesem is only rabbinic, kusim shouldn't hold of it. Tosafos answers that they assume that it comes from her body. Meaning, that the kusim don't follow the rabbinic parameters of kesamim to be strict or lenient. Rather, they follow their own simplistic assumptions that it would have come from her body. It should follow that kusim not only keep the laws of kesamim but they are stricter than we are and render even kesamim smaller than a gris or found on something not susceptible to tu'mah, to make her tamei. This can actually lead to a problem since the Kusi may start the count of 7 days from a kesem that didn't make her tamei, completely throwing off her proper day of immersion.